The quasi-stationary feature of nocturnal precipitationin the Sichuan Basin and the role of the Tibetan Plateau

Xia Jin • Tongwen Wu • Laurent Li

Received: 7 May 2012 / Accepted: 4 September 2012 / Published online: 25 September 2012

� The Author(s) 2012. This article is published with open access at Springerlink.com

Abstract The nocturnal precipitation in the Sichuan

Basin in summer has been studied in many previous works.

This paper expands the study on the diurnal cycle of pre-

cipitation in the Sichuan Basin to the whole year. Results

show that the nocturnal precipitation has a specific quasi-

stationary feature in the basin. It occurs not only in summer

but also in other three seasons, even more remarkable in

spring and autumn than in summer. There is a prominent

eastward timing delay in the nocturnal precipitation, that is,

the diurnal peak of precipitation occurs at early-night in the

western basin whereas at late-night in the center and east of

the basin. The Tibetan Plateau plays an essential role in the

formation of this quasi-stationary nocturnal precipitation.

The early-night peak of precipitation in the western basin is

largely due to strong ascending over the plateau and its

eastern lee side. In the central and eastern basin, three

coexisting factors contribute to the late-night peak of pre-

cipitation. One is the lower-tropospheric southwesterly

flow around the southeastern edge of the Tibetan Plateau,

which creates a strong cyclonic rotation and ascendance in

the basin at late-night, as well as brings abundant water

vapor. The second is the descending motion downslope

along the eastern lee side of the plateau, together with an

air mass accumulation caused by the warmer air mass

transport from the southeast of the Yunnan-Guizhou Pla-

teau, creating a diabatic warming at low level of the tro-

posphere in the central basin. The third is a cold advection

from the plateau to the basin at late-night, which leads to a

cooling in the middle troposphere over the central basin.

All these factors are responsible for precipitation to occur

at late-night in the central to eastern basin.

Keywords Quasi-stationary � Diurnal cycle � Nocturnal

precipitation � Sichuan Basin � Tibetan Plateau

1 Introduction

The diurnal variation of precipitation is a significant aspect

of weather and climate. It is often related to physical

processes of local geography or regional atmospheric

dynamics (e.g. Liu and Moncrieff (1998) and Lin et al.

(2000) for a review on this topic). It is well known that the

complex geographical environment in the Asian monsoon

region can exert strong influences on the regional climate.

Numerous studies have shown that there are remarkable

diurnal characteristics of the precipitation in summer in the

Asian monsoon region. Such studies use either rain-gauge

measurements (e.g. Fujinami et al. 2005; Zhao et al. 2005;

Yu et al. 2007a, b; Li et al. 2008) or satellite data (Zhou

et al. 2008; Chen et al. 2009; Yu et al. 2009; He and Zhang

2010). The nocturnal precipitation in the Sichuan Basin in

summer, i.e. the precipitation reaches its diurnal peak

around midnight, becomes a hot topic in the recent decade

(Li et al. 2008; Yu et al. 2007a, 2009; Chen et al. 2010;

Huang et al. 2010; Bao et al. 2011; Yuan et al. 2011). This

phenomenon has been observed and documented since long

time (Lu 1942; Ye and Gao 1979; Zeng et al. 1994). But

X. Jin

Chinese Academy of Meteorological Sciences, China

Meteorological Administration, Beijing, China

X. Jin � L. Li

Laboratoire de Meteorologie Dynamique, IPSL, CNRS/UPMC,

Paris, France

X. Jin � T. Wu (&) � L. Li

National Climate Center, China Meteorological Administration,

46 Zhongguancun Nandajie, Beijing 100081,

People’s Republic of China

e-mail: twwu@cma.gov.cn

123

Clim Dyn (2013) 41:977–994

DOI 10.1007/s00382-012-1521-y

only a few recent works have addressed the physical

mechanisms in controlling the diurnal precipitation varia-

tion in summer in the Sichuan Basin (e.g. Dai et al. 2007;

Yu et al. 2007a). Yu et al. (2007b) and Li et al. (2008)

showed that the summertime nocturnal precipitation in the

Sichuan Basin is contributed mainly by long-duration

precipitating systems. In addition, the diurnal cycle of

precipitation presents an eastward delayed diurnal phase

from the east of the Tibetan Plateau (TP) down to the

Yangtze River valley in central China (Chen et al. 2010;

Huang et al. 2010; Bao et al. 2011; Yuan et al. 2011).

Several works proposed their assumptions to explain the

summertime nocturnal precipitation in the Sichuan Basin.

Li et al. (2003, 2008) suggested that it may be caused by a

weakening of the mid-level inversion layer at night in

summer. A numerical modeling study by Huang et al.

(2010) on an individual event revealed that the eastern TP

acts as a heat source for convection and a thermal driven

diurnal solenoid circulation results in an ascendance over

the Sichuan Basin in nighttime, which favors the nocturnal

precipitation. And the diurnal variation of the solenoid

circulation may contribute to the longevity and propagation

of episodes in the vicinity of the eastern TP. Bao et al.

(2011) employed the theory of mountain-plain solenoid

propagation (may refer to Tripoli and Cotton 1989; Car-

bone and Tuttle 2008) to understand the phenomena of

eastward delayed phase for precipitation in the Sichuan

Basin. Chen et al. (2010) and Yuan et al. (2011) suggested

that the summertime nocturnal precipitation was attributed

to the decreasing stability caused by the low-tropospheric

convergence and upward motion, accompanied by a long

wave radiative cooling at the cloud top and the weak cold

advection from the TP. These works have shown that the

diurnal cycle of precipitation in summer is intimately

linked to the topography in southwestern China.

Most previous studies on the diurnal variation of pre-

cipitation have focused on summer (June to August), the

main rainy period in central China. However, the precipi-

tation in the other seasons also accounts for a significant

portion around the year. The regional average precipitation

rates in the Sichuan Basin in spring (March to May) and

autumn (September to November) account about 23 % of

the total annual precipitation. In winter (December to

February), the precipitation frequency is over 11 % (not

shown). The purpose of this work is to explore the feature

of diurnal cycle of precipitation in the Sichuan Basin in all

the seasons and discuss the physical mechanisms involved

in the diurnal cycle of precipitation.

The rest of the paper is organized as follows: Sect. 2

describes the dataset used in this work. Section 3 presents

the characteristics of the diurnal precipitation cycle in the

Sichuan Basin in different seasons. Section 4 analyzes the

dynamic and thermodynamic effects of the TP on the

diurnal cycle of precipitation in the Sichuan Basin. Con-

clusions are summarized in Sect. 5.

2 Data

The hourly 0.1� 9 0.1� resolution merged precipitation

products cover whole China from January 2008 to

December 2010 (Yu et al. 2011; Shen et al. 2012). This

dataset, produced by the National Meteorological Infor-

mation Center of China Meteorological Administration, is

used to study the diurnal cycle of precipitation. It combines

the National Oceanic and Atmospheric Administration

(NOAA)/Climate Prediction Center morphing technique

(CMORPH) precipitation data (Joyce et al. 2004) and the

hourly precipitation dataset observed by more than thirty

thousand automatic meteorological stations in China

(Fig. 1) (Shen et al. 2010a). As a high spatial/temporal

precipitation dataset, CMORPH has better performance

than many other satellite products over China (Shen et al.

2010b). However, the CMORPH dataset has systematic

errors over China, characterized by an underestimation of

strong precipitation and an overestimation of weak pre-

cipitation (Yu et al. 2011). So the ground observation

precipitation dataset is used to correct the CMORPH data.

The procedure to produce the merged precipitation prod-

ucts has mainly three steps: Firstly, the irregularly-dis-

tributed and quality-controlled hourly precipitation data

derived from automatic meteorological stations are inter-

polated spatially into 0.1� 9 0.1� gridded data through an

optimal interpolation based on the climatological back-

ground (Shen et al. 2010a). Secondly, the 30-min 8-km

resolution CMORPH satellite precipitation data are inter-

polated into hourly interval and 0.1� 9 0.1� grid. Finally,

systematic errors of CMORPH with respect to the ground

observed precipitation are estimated to correct the

CMORPH satellite precipitation. The systematic error is

defined as a difference between the precipitation amounts

corresponding to the same cumulative probability density

for both datasets at every 0.1� longitude/latitude grid (Yu

et al. 2011). As shown in Fig. 1, there are abundant auto-

matic meteorological stations distributed in the Sichuan

Basin where the precipitation observations have been

involved in the merged precipitation products. This dataset

is believed to have a very good quality in the Sichuan

Basin.

In order to characterize the atmospheric dynamical and

thermo-dynamical states which are in close relationship

with the diurnal behavior of precipitation, we use a subset

of the ERA-Interim reanalysis data (Dee et al. 2011). This

dataset has a 0.75� 9 0.75� resolution and outputs at 6 h

interval at 00, 06, 12 and 18 UTC. The period from January

2003 to December 2010 is used in this study.

978 X. Jin et al.

123

3 Characteristics of diurnal cycle of precipitation

The diurnal peaks of seasonally-averaged hourly precipi-

tation frequency and rate for four seasons over the south of

China are shown in Figs. 2 and 3, respectively. The

methodology used here is based on an explicit maximum

selection. Hourly period in local solar time (LST) during

which the precipitation frequency (or rate) reaches its

maximum in 1 day at each 0.1� 9 0.1� grids are marked.

Light colors stand for daytime peaks whereas dark colors

are related to nighttime peaks.

As shown in Fig. 2, in the Sichuan Basin (encircled in a

red circle), the maximal precipitation frequency occurs at

nighttime (1800–0600LST) in all the seasons, which is

remarkably different from an afternoon (1200–1800LST)

frequency peak in most other areas of the continent. In

spring (Fig. 2b), the area with nocturnal peaks of precipi-

tation frequency is most pronounced among these four

seasons. It covers the whole Sichuan Basin and a large area

to the east of the Yunnan-Guizhou Plateau (YGP), which is

Fig. 1 Terrain distribution (unit: m; contoured in black solid) over

the South of China and density distribution of the automatic

meteorological stations (colored; different colors denote number of

stations in each 0.1� 9 0.1� grid area). The Yangtze River and the

Yellow River are plotted in blue. TP and YGP are short for the

Tibetan Plateau and the Yunnan-Guizhou Plateau, respectively. The

Sichuan Basin is marked with a red circle

(a) (b)

(d)(c)

Fig. 2 Local solar time (LST) of diurnal peaks for precipitation

frequency as derived from hourly merged precipitation products at

each 0.1� 9 0.1� grid for the seasons of a December–January–

February (DJF), b March–April–May (MAM), c June–July–August

(JJA) and d September–October–November (SON) from 2008 to

2010. The Yangtze River and the Yellow River are plotted in blue.

The Sichuan Basin is marked with a red circle

The quasi-stationary feature 979

123

located in the southwest of the Sichuan Basin (as shown in

Fig. 1). It even spreads to the south of the YGP. In autumn

(Fig. 2d), the area with the highest precipitation frequency

at night is not as large as in spring but still significant. It

covers most of the basin and the region to the eastern slope

of the YGP. In winter (Fig. 2a), the nocturnal peaks of

precipitation frequency still exist in the basin, but is mainly

located in the south of the basin, the east of the YGP and

the coastal region of South China. In summer (Fig. 2c), the

area with the most frequent nocturnal precipitation is

mainly located inside the basin, which is smaller than that

in spring and autumn. We also note that in the eastern

basin, the peak of diurnal cycle of precipitation frequency

in summer has its specificity and appears in afternoon but

not at nighttime. It is possibly caused by a stronger local

mountain heating in the afternoon of summer season. No

matter how the area varies by season, the maximal pre-

cipitation frequency in the Sichuan Basin generally appears

at nighttime through the whole year.

It is noted that the color transition from brown to green

in the Sichuan Basin exhibits an eastward phase delay for

the maximal precipitation frequency in all seasons (Fig. 2),

which is significant especially in spring and autumn.

In spring, the maximum precipitation frequency timing

shows a highly regular transition from west to east in the

basin. That is, the early-night (1800–0000LST) peaks

appear in the western basin (west of 105�E) while the late-

night peaks (0000–0600LST) always occur in the central to

eastern basin (east of 105�E). This phenomenon can also be

observed in autumn and summer. In winter, an eastward

delay still exists, although the area with frequent nocturnal

precipitation has moved to the south of the basin.

The geographic distribution of peak timing for season-

ally-averaged hourly precipitation rate (Fig. 3) in the

Sichuan Basin shows similar spatial features as for pre-

cipitation frequency (Fig. 2). The diurnal maximal pre-

cipitation occurs at nighttime in the basin and also presents

a longitudinal phase-delay in most of the seasons. The

discrepancy between west and east of the basin is obvious

in spring, summer and autumn. The diurnal variation of

precipitation rate peaks at early-night (1800–0000LST)

over most of the western basin and at late-night

(0000–0600LST) in the central and eastern basin, respec-

tively. In winter, this phenomenon is not as conspicuous as

in other three seasons whereas it can still be distinguished.

In addition, some differences in summer are observed in

(a) (b)

(d)(c)

Fig. 3 Same as in Fig. 2 but for precipitation rate

980 X. Jin et al.

123

Fig. 3c with respect to Fig. 2c. In the southeastern part of

the basin, the maximal precipitation rate appears in the

morning (0600–1200LST), whereas the maximal precipi-

tation frequency occurs in the afternoon (1200–1800LST).

Figure 4 shows the Hovmoller diagram of seasonally

mean of hourly precipitation for the latitude band between

28�N and 32�N. There exists an obvious eastward timing

delay for precipitation maximum from the east of the TP

(about 93�E–100�E) to the eastern plain in four seasons. A

propagation of precipitation seems to originate from the TP

and moves eastward to the Sichuan Basin. However, when

we view all individual precipitation events in the period

from January 2008 to December 2010, it is found that most

of them are originated from the basin (for example in

Figs. 5a–c). Only few cases of precipitation begin from

the TP and then propagate eastward to the basin in summer

(a)

(b)

(c)

(d)

Fig. 4 Time-longitude

diagrams of hourly precipitation

(mm h-1) averaged between

28�N and 32�N for a DJF,

b MAM, c JJA and d SON.

Vertical axis indicates time in

UTC. Diagonal lines are lines of

equal local solar time

The quasi-stationary feature 981

123

(for example in Fig. 5d). If we average these four cases of

precipitation shown in Fig. 5, it would show a continuous

precipitation process from the TP to the eastern basin

which is similar to the pattern shown in Fig. 4.

4 Role of the TP in the quasi-stationary nocturnal

precipitation

Although the summertime nocturnal precipitation in the

Sichuan Basin has been explored in many previous studies,

the above analyses further show that the nocturnal peak of

diurnal precipitation cycle exists steadily in all seasons,

and the nocturnal precipitation is even more significant in

spring and autumn than in summer. The quasi-stationary

behavior of the nocturnal precipitation peak gives us the

intuition that it must be strongly related to the topography

of the region.

The Sichuan Basin is adjacent to the eastern side of the

TP and the northeastern slope of the YGP. To its east, the

basin is surrounded by a series of small mountains with an

average altitude of 1,000 m (Fig. 1). The TP and the YGP

are the most significant highlands over the southwest of

China. The main body of the TP has an average elevation

of over 4,000 m above sea level (near 600 hPa). The YGP,

which is located in Yunnan and Guizhou provinces in

Southwestern China and adjacent to the southeast of the

TP, is over 2,000 m in elevation. The area of YGP is much

(a) (b)

(c) (d)

Fig. 5 Time-longitude diagrams of hourly precipitation (mm h-1)

averaged between 28�N and 32�N for a 0600UTC 20th—0000UTC

22th, April 2008, b 0600UTC 22th—0600UTC 24th, April 2009,

c 0000UTC 10th—1800UTC 11th, April 2010 and d 0600UTC

27th—0000UTC 29th, August 2008

982 X. Jin et al.

123

smaller than that of the TP, and the altitude of the YGP

decreases gradually from northwest to southeast, which

looks like a stretch of the southeast of the TP, so hereafter

we put the YGP together with the TP as a united plateau for

the convenience of discussion. In this section, two main

aspects are analyzed: dynamical forcing and thermal

dynamical effect of the plateau. In this section, the LST is

according to the center of the Sichuan Basin (30�N,

105�E).

4.1 Influences of large-scale dynamical forcing

of the plateau

Figure 6 depicts the profile of climatological mean relative

vorticity and vertical velocity along the latitude of 30�N. On

the impact of the TP terrain, the lower troposphere in the

southeast periphery of the TP is dominated by the

southwesterly wind in DJF, MAM and JJA and the south-

easterly wind in SON. A steady deep cyclonic vorticity forms

over the eastern slope of the TP. The strong positive vorticity

dominates the low level of the troposphere (from surface to

750 hPa) in the central and eastern basin (from 105�E to

108�E). It stretches upward to the middle level of the tro-

posphere (approximately 500 hPa). This quasi-stationary

positive vorticity in the center and east of the Sichuan Basin

is well correspondent to the upward motion which favors

atmospheric instabilities and a center of precipitation in the

Sichuan Basin (not shown) in all seasons.

The cyclonic vorticity in the basin has a conspicuous

diurnal variation. Figure 7 describes the diurnal cycle of

relative vorticity and wind at 800 hPa level in different

seasons. The positive relative vorticity in the central basin

is apparently stronger at nighttime than at daytime in all

seasons, which is in accord with the local ascendance and

(a) (b)

(d)(c)

Fig. 6 Profile of seasonal mean relative vorticity (10-5 s-1, shaded) and vertical velocity (Pa s-1, contour) along the latitude of 30�N for a DJF,

b MAM, c JJA and d SON. The interval of contours is 0.04 Pa s-1. Topography is shaded in white

The quasi-stationary feature 983

123

Fig. 7 Diurnal cycle of relative vorticity (10-5 s-1, shaded) and

wind (m s-1, vector) at 800 hPa for the seasons of (top row) DJF,

(second row) MAM, (third row) JJA and (bottom row) SON. From left

to right, the columns are for 1900LST, 0100LST, 0700LST and

1300LST, respectively. The 2,000 m height contour is sketched with

grey thick line. The Sichuan Basin is marked with a black circle

984 X. Jin et al.

123

precipitation at late-night. The strongest positive relative

vorticity almost occurs at 0100LST, while the weakest one

appears at 1300LST. In spring and summer, the strength of

this positive relative vorticity varies most significantly, as

the diurnal variation range achieves 2 9 10-5 s-1.

The diurnal variation of the relative vorticity in the

central basin is intimately associated with that of the wind.

As shown in Fig. 7, the southwesterly wind in spring,

summer and winter prevails in southeast of the TP. It flows

around the southeastern periphery of the YGP and enters

(a) (e)

(f)

(g)

(h)

(b)

(c)

(d)

Fig. 8 Correlation between regional averaged relative vorticity in the

Sichuan Basin and (left column) zonal wind field and (right column)

meridional wind field over 800 hPa at 0100LST for a, e DJF,

b, f MAM, c, g JJA and d, h SON. The correlation coefficient over 0.2

is significant at the 99.9 % confidence level as determined by the

Student’s t test. The Sichuan Basin is marked with a red circle

The quasi-stationary feature 985

123

the eastern part of the basin through the lower mountain in

the southeast of the basin. Due to the topographic forcing

of the concave basin, this flow causes a strong cyclonic

rotation in the central basin. In autumn, although south-

easterly flow dominates a large area over the southeast of

the TP, by the obstruction of the YGP, it turns its direction

to north and flows around the eastern periphery of the YGP

and goes into the basin, and finally forms a cyclonic

rotation similar to that in other three seasons in the central

basin.

Figure 8 gives a clear association between the regional

average relative vorticity in the basin and gridded zonal/

meridional wind velocity over East Asia at 800 hPa for

0100LST. The spatial correlations shown in Figs. 8a–d

presents that the relative vorticity in the Sichuan Basin is

positively correlated to the westerly wind over a large area

in the southeast periphery of the YGP. Especially in spring

(Fig. 8b) and winter (Fig. 8a), the region of high positive

correlation coefficients is located in the south and southeast

slope of the YGP where the westerly wind prevails. As a

result of the cyclonic rotation in the basin, positive

correlation is found in south of the basin and negative

correlation in north of the basin in the four seasons. In

Fig. 8e–h, a large area of positive correlation between

relative vorticity in the basin and the gridded meridional

wind velocity is located in the southeast slope of the YGP

in all the seasons, which denotes that the strengthening of

the southerly wind in this region has a strong relationship

with the increasing positive relative vorticity in the central

basin at late-night.

Figure 9 shows the spatial distribution of vectors (A(i, j),

B(i, j)), whose components are the coefficients of dualistic

linear regression (vortbasin ¼ Aði;jÞ � uði;jÞ þ Bði;jÞ � vði;jÞþCði;jÞ) of the regional averaged relative vorticity in the

Sichuan Basin (vortbasin) on the horizontal wind (u(i, j), v(i,

j)) at each grid over 800 hPa for 0100LST. The direction of

vector at each grid denotes the wind direction exerting

influence on the relative vorticity in the basin, and the

length of the vector quantifies the intensity. It shows high

consistency in all of the seasons. Long vectors are on the

slope of the mountain around the basin and in the south-

eastern periphery of the YGP. Vectors surrounding the

(a) (b)

(d)(c)

Fig. 9 The spatial distribution of vectors (A(i, j), B(i, j)), whose

components are the coefficients of dualistic linear regression

(vortbasin ¼ Aði;jÞ � uði;jÞ þ Bði;jÞ � vði;jÞ þ Cði;jÞ) of the regional aver-

aged relative vorticity in the Sichuan Basin (vortbasin) on the

horizontal wind (u(i, j), v(i, j)) at each grid over 800 hPa for

0100LST for a DJF, b MAM, c JJA and d SON. The unit is m-1.

The Sichuan Basin is marked with a red circle. The area (24�N–28�N,

107�E–112�E) is marked with a red rectangle

986 X. Jin et al.

123

basin turn cyclonically due to orographic forcing. The

southerly and southwesterly winds dominate the region in

the southeastern side of the YGP and play an important role

on the variation of relative vorticity in the central basin at

late-night in four seasons.

As illustrated in Figs. 8 and 9, it implies that the diurnal

variation of the southwesterly wind around the southeast

vicinity of YGP has an essential role on the diurnal cycle of

the low-tropospheric relative vorticity in the basin. Fig-

ure 10 presents the diurnal cycle of averaged meridional

wind over the southeast periphery of YGP (24�N–28�N,

107�E–112�E). It shows a positive center of southerly wind

at low level of the troposphere (from 850 hPa to 750 hPa)

at 0100LST in different seasons, which exactly corre-

sponds to the strongest positive relative vorticity in the

basin (Fig. 7).

In addition, the low-tropospheric southwesterly wind

also brings more moisture at night, which favors the noc-

turnal precipitation. Figure 11 shows the climatological

background of meridional water vapor transport in the

atmospheric column from surface to 500 hPa for four

seasons. Accompanying the low-level southwesterly,

moisture from the south enters the eastern basin through its

opening in the southeast in all seasons. It has an effect to

the precipitation in the central and eastern basin. Note that

there is less water vapor transport directly from the south in

the western part of the Sichuan Basin because of the terrain

obstruction. It seems that the air flow coming from the

southeast periphery of the TP can not directly affect the

rainfall in the western part of the basin.

Significant differences between west and east of the

basin also exist in the diurnal variation of the atmospheric

water vapor content integrated from surface to 500 hPa. As

shown in Fig. 12, the maximal water vapor content in the

western basin (102�E–104�E) appears at early-night (1800–

0000LST) in all the seasons, which is beneficial for early-

night precipitation. However, in the central and eastern

basin (105�E–108�E), the maximal water vapor content

(a) (b)

(d)(c)

Fig. 10 Diurnal cycle of regionally-averaged meridional wind (m s-1) in the area (24�N–28�N, 107�E–112�E) as marked in Fig. 9 for the

seasons of a DJF, b MAM, c JJA and d SON. The horizontal axis signifies the LST and the vertical axis denotes the barometric height (hPa)

The quasi-stationary feature 987

123

occurs at late-night (0000–0600LST), favorable for pre-

cipitation after midnight.

4.2 Thermal dynamical effect of the TP

The Sichuan Basin is located in the downstream of the

westerly wind from the TP. The atmospheric circulation over

the TP has an important influence on the diurnal variation of

precipitation in the Sichuan Basin, which is closely related to

the atmospheric temperature variation over the TP.

The diurnal variation of potential temperature over

550 hPa, which is close to the surface of the TP, is shown

in Fig. 13. Although potential temperature experiences

considerable seasonal variation, it still varies regularly at

diurnal scale. It is evident that the strongest diurnal

anomalous potential temperature (both positive and nega-

tive values) occurs over the TP. The potential temperature

near the surface of the TP at daytime is obviously warmer

than that during nighttime. The warmest near-surface

temperature occurs at 1900LST and the coldest at

0700LST. After sunset (near 1900LST), the near-surface

potential temperature drops dramatically and continues to

decrease during the whole night until the sunrise (near

0700LST). The diurnal variation of atmospheric potential

temperature over the TP ranges between ?2 and -2 K in

contrast to the daily mean. However, the potential tem-

perature at 550 hPa in the basin varies much more mod-

erately, as the variation range is only between ?0.5 and

-0.5 K.

Such a significant diurnal variation of potential tem-

perature over the TP has a great influence on the atmo-

spheric circulation. As presented in Fig. 13, at daytime, the

warming over the TP causes arising motion and conver-

gence near the surface of the TP. The strongest conver-

gence appears at 1900LST. During nighttime, the

continuous cooling over the TP leads to subsidence and

(a) (b)

(d)(c)

Fig. 11 Seasonally-mean and vertically-integrated meridional water vapor transport from surface to 500 hPa (R 500

surfqvdp, unit: 102 kg s-3) for

a DJF, b MAM, c JJA and d SON. The Sichuan Basin is marked with a black circle

988 X. Jin et al.

123

divergence over the near-surface of the TP. As the surface

of the TP is colder and colder, the strengths of the subsi-

dence and the divergence are increasing during the whole

night and reach their strongest values at 0700LST.

Figure 14 presents the vertical profile of diurnal cycle of

anomaly from daily mean of the potential temperature and

circulation in the cross section along 30�N. It is clear that at

1900LST, the ascendance caused by strong warming over

the TP dominates the TP and its eastern vicinity. The

western basin is controlled by upslope wind at early-night,

which is favorable for precipitation. At the same time, there

is a compensating descending motion suppressing precip-

itation in the center and east of the basin, as the lower-

troposphere water vapor source in the western basin mainly

comes from the central to eastern basin and less water

transporting directly from the south (Fig. 11). They con-

struct a solenoidal circulation at the lower to middle level

in the basin. At 0100LST, as the temperature drops over the

near-surface of the TP, descending motion controls the TP.

The downdraft flow downslope along the eastern lee side of

the plateau to the central basin. There is a relatively war-

mer center in the central basin at lower level of the tro-

posphere (from 900 to 800 hPa). It is possibly caused by

the diabatic descent warming and accumulation of warmer

air mass coming from the southeastern edge of the YGP in

the basin. This anomalous warm air mass is easily to cause

the middle troposphere (about 800 hPa) getting more

instability at late-night than other times. The relatively

warm air arises in the central and eastern basin and the

downslope flow in the western basin construct an anoma-

lous solenoidal circulation at the lower- to mid-troposphere

in the basin.

As shown in Fig. 14, along with the development of

cooling over the TP at late-night, the anomalous downward

motion near the surface of the TP is more and more

intensified and even spreads to the mid-troposphere in the

(a)

(c) (d)

(b)Fig. 12 Time-longitude

diagrams of the anomaly from

daily mean of integrated specific

humidity from surface to

500 hPa (R 500

surfqdp, unit: 102

kg m-1 s-2) averaged from

28�N to 32�N for a DJF,

b MAM, c JJA and d SON. The

vertical axis signifies the LST

The quasi-stationary feature 989

123

basin. In addition, the hill in the east of the basin also gets

cooling before sunrise. The anomalous ascending motion at

late-night in the low hill in the east of the basin is gradually

changed to anomalous descending motion before sunrise.

After sunrise, the anomalous descending motion in the east

of the basin is gradually replaced by ascending motion.

As the altitude of the TP is above 600 hPa, the strong

cooling and divergence near the surface of the TP at night

cause an eastward cold advection from the plateau and

leads to a cooling at the middle troposphere in the basin.

Figure 15 shows the diurnal cycle of zonal advection of

potential temperature at 550 hPa at the location of the

central basin (30�N, 105�E). In winter, spring and autumn,

there is a cold advection after midnight. In summer, the

cold advection is not as strong as in other seasons. But the

warm advection is very weak (almost zero) at late-night.

This eastward cold advection at middle level of the tro-

posphere encounters the warmer updraft with abundant

Fig. 13 Anomalous (by the daily mean) diurnal cycle of potential

temperature (K, shaded) and divergence (10-6 s-1, contour) at

550 hPa for (top row) DJF, (second row) MAM, (third row) JJA and

(bottom row) SON. From left to right, the columns are for 1900LST,

0100LST, 0700LST and 1300LST, respectively. The contour intervals

are 5 9 10-6 s-1. The Sichuan Basin is marked with a black circle

990 X. Jin et al.

123

Fig. 14 Profile of diurnal variation of anomalous vertical circulation

(zonal wind and 10 times of vertical velocity, streamline) and

anomalous potential temperature (K, shaded) along the latitude of

30�N for (top row) DJF, (second row) MAM, (third row) JJA and

(bottom row) SON from their daily means. From left to right, the

columns are for 1900LST, 0100LST, 0700LST and 1300LST,

respectively. The vertical axis denotes pressure level (hPa)

The quasi-stationary feature 991

123

water vapor from lower level in the central and eastern

basin, favorable for the occurrence of nocturnal

precipitation.

5 Summary and conclusion

Many previous studies focus on the nocturnal precipitation

in the Sichuan Basin in summer. This work expands the

study on the diurnal cycle of precipitation in the Sichuan

Basin to four seasons of the whole year. Results show that

nocturnal precipitation has a quasi-stationary feature in the

basin during the whole year. It appears not only in summer

but also in other three seasons. In particular, the areas with

nocturnal maximal precipitation in spring and autumn are

more remarkable than that in summer. There is a prominent

Fig. 15 The diurnal cycle of zonal advection of potential temperature

(�u ohox

, unit: 10-3 K s-1) at 550 hPa over the position (30�N, 105�E)

for the seasons of DJF, MAM, JJA and SON. The horizontal axis

signifies the LST

Fig. 16 Schematic diagram

illustrating the different factors

on the nocturnal precipitation at

a early-night and b late-night.

The green diagonal shade

denotes the area where the

diurnal phase of precipitation

peaks at each time

992 X. Jin et al.

123

eastward timing delay of the diurnal precipitation peak

between the western basin and the central and eastern

basin. That is the maximal precipitation occurs at early-

night (1800–0000LST) in the western basin, but appears at

late-night (0000–0600LST) in the central and eastern basin.

Most of the nocturnal convection along 30�N might be

caused by local forcing, and there is rarely propagation of

the convection originated from the plateau.

The TP plays an essential role in the formation of this

quasi-stationary nocturnal precipitation in the Sichuan

Basin. To summarize, a schematic diagram is shown in

Fig. 16, the rainfall in western basin is much closely linked

to the thermal condition of the plateau plain. At early-night

(Fig. 16a), the western basin is dominated by upward

motion which is caused by a strong warming at the near

surface of the TP. The strong warming over the TP leads to

ascendance over the TP and upslope wind along the eastern

lee side of the TP, favoring strong precipitation at early-

night in the western basin.

The physical mechanism for nocturnal precipitation in

the central and eastern basin is different from that in the

western basin. There are three coexisting factors contrib-

uting to the late-night peak of precipitation in the central

and eastern basin (Fig. 16b):

1. There is a strengthening southwesterly air current

flowing around the southeastern edge of the YGP at

low level of the troposphere at late-night than in other

times. It enters the eastern part of the basin and causes

a stronger cyclonic rotation in the central basin,

favoring ascendance at late-night. This air current

from south also brings abundant water vapor into the

basin, which is propitious for nocturnal precipitation.

2. After sunset, the atmosphere at near surface of the TP

cools down substantially, creating cold downdraft and

divergence over the TP. There is an accumulation of

warmer air mass in the central basin, which comes

from the southeastern edge of the YGP. As the cold air

over the TP moves downslope along the eastern lee

side of the TP to the central basin, it causes a diabatic

warming at low level of the troposphere (from 900 to

800 hPa) in the basin. Then warm air arises in the

center and east of the basin, creating atmospheric

instability.

3. The cooling and strong divergence over the TP causes

an eastward cold advection at middle level of the

troposphere in the central basin at late-night. The

lower level warm updraft encounters this cold advec-

tion in the central and eastern basin, which eases the

occurrence of nocturnal precipitation.

Future studies will examine the respective contributions

of different mechanisms involved in the nocturnal precip-

itation in the Sichuan Basin. We are also planning to

examine how climate models involved in future climate

projection reproduce this phenomenon of nocturnal pre-

cipitation in the Sichuan Basin and the associated physical

mechanisms.

Acknowledgments This work was supported by National Basic

Research Program of China (973 Program:2010CB951902) and

Special Program for China Meteorology Trade (Grant No.

GYHY200806006). The merged precipitation products was supplied

by National Meteorological Information Center/China Meteorological

Administration. The ERA-Interim dataset was provided by European

Centre for Medium-Range Weather Forecasts (ECMWF).

Open Access This article is distributed under the terms of the

Creative Commons Attribution License which permits any use, dis-

tribution, and reproduction in any medium, provided the original

author(s) and the source are credited.

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